This invention relates to a ball joint which is used for such purposes as rotatably securing a piston end and a cylinder end of a piston-cylinder type air spring for coupling a vehicle body and a hatch door of a hatch-back type vehicle to the vehicle body and the door.
In order to permit smooth opening and closing of a hatch door of a hatch-back type vehicle and also permit the door to be suitably held open, an air spring or like coupler is provided between the vehicle body and the hatch door. The air spring consists of a cylinder and an extensible piston rod accommodated therein. This air spring adopts a ball joint structure for the portions where it is coupled to the vehicle body and the door.
Such a ball joint is disclosed in Japanese Patent Application Disclosure SHO No. 59(1984)-17017. The disclosed ball joint comprises a socket and a ball stud having a ball accommodated in a accommodating section formed in the socket as a vertical passage. In the accommodating section is also accommodated an elastic ring-like member which has an inner diameter smaller than the diameter of the equator of the ball and is cut away in a portion of the circumference thereof to form a gap therein. Further, a cap is screwed in the accommodating section from above to cover substantially the upper half of the ball. In this state, the portion of the ball below the equator is held by the elastic ring-like member.
In the above structure, as the cap is screwed down, its lower end downwardly urges the elastic ring-like member to prevent spreading thereof. Therefore, when disassembling the socket and ball stud, it is necessary to loosen the cap by turning it several rotations so that the lower end of the cap is separated from the elastic ring-like member, which thus can be spread. The fastening of the cap by downwardly screwing it and also its loosening by unscrewing it are rather cumbersome and time-consuming.